1. Field of the Invention
The present invention relates to the construction of a sensor for converting physical parameters into electrical signals.
2. Prior Art
Various strain gauges are known. For example, there has been described, in Sensors and Actuators, Vol. 2, (1981/82) 17-27, by Prudenziati, et al, the properties of strain gauges made from series ESL 3100 and DuPont series DP 7600 inks, fired onto enameled steel substrates. Such structures have appreciable, and rather irreproducible, temperature coefficients of resistance. This makes them difficult to use in the form of a temperature compensated strain gauge bridge and for the purpose of measuring slowly changing stresses or pressures. On the other hand, such structures can tolerate relatively high temperatures, such as those encountered by a combustion pressure sensor attached to the head of an internal combustion engine.
It is conventional to fit pressure diaphragms with two matched strain gauges, one in a position of maximum tensile strain, the other in a position of maximum compressive strain. Two such strain gauges then form part of a four-resistor bridge. The remaining two bridge resistors are constant, external resistors. The output of such a bridge is temperature compensated, given the following assumptions: (1) both strain gauges have precisely the same temperature coefficient of resistivity, (2) the diaphragm is isothermal, and (3) there either is no aging of either of the two strain gauge resistors, or else they age in precisely the same manner.
U.S. Pat. No. 4,217,783 issued to Ito et al teaches a magnetoresistive pressure-sensing device for automotive electronic engine control systems. The sensor is formed on a glass diaphragm by ordinary thin film techniques and produces an electrical resistance change dependent on the deflection of the diaphragm. It is expected that such thin alloy films would be unsuitable for elevated temperatures of combustion. For example, it would be expected that considerable sintering of films that thin would occur and that there would be consequent changes of film characteristics.
U.S. Pat. No. 3,697,917 issued to Orth et al teaches a method for making a semiconductor strain gauge pressure transducer. Multiple strain gauges are formed on a silicon diaphragm and connected in a Wheatstone bridge circuit arrangement in which resistance changes are measured as an indication of pressure changes. Again, such single crystal silicon piezoresistive elements are generally unsuited for high temperature applications. In contrast, they are more typically used for applications such as measuring barometric pressure and intake manifold pressure.
U.S. Pat. No. 4,169,388 to Teitelbaum et al teaches a piezoelectric technique for measuring pressure. Again, the disclosed structure has shortcomings, when used in a high temperature environment, such as the direct temperature sensitivity of the piezoelectric material, the indirect temperature effects due to the strains of the differential thermal expansion in the mount for the piezoelectric material, the general structural friability of the piezoelectric ceramic, and the high source impedance of all piezoelectric devices.
It would be desirable to have a simple, relatively inexpensive sensor which could operate in a high temperature environment and measure the combustion pressure in a cylinder of an internal combustion engine. These are some of the problems this invention overcomes.